Diffusion blocking layer for a compound semiconductor structure

ABSTRACT

A method of fabricating a gain medium includes growing a p-type layer doped with zinc on a substrate, growing an undoped layer including one or both of InP or InGaAsP on the p-type layer, growing a region that includes multiple quantum wells (MQWs) on the undoped layer, and growing an n-type layer on the region. The undoped layer has a thickness that is sufficient to prevent Zn diffusion from the p-type layer into the region during subsequent growth or wafer fabrication steps.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application of, and claims thebenefit of priority of, U.S. Provisional Patent Application Ser. No.62/693,626, filed 3 Jul. 2018, which is incorporated by referenceherewith in its entirety for all purposes.

BACKGROUND

Compound semiconductors that have direct bandgaps are important inelectronics, because they can respond to electrical stimuli by emittinglight, and/or respond to receiving light by emitting electrical signals.Well-known compound semiconductors include III-V system materials thatuse one or more group III materials such as Ga, In or Al, and one ormore group V materials such as N, P As or Sb. II-VI materials are alsopossible, with one or more group II materials such as Zn, Cd or Hg, andone or more group VI materials such as S, Se or Te. In the III-Vsystems, Zn, Cd or Mg are sometimes used as p-type dopants while Si issometimes used as an n-type dopant.

SUMMARY

Systems and methods herein recognize the advantages of providingcompound semiconductors with an undoped layer between one or more p-typelayers doped with zinc, and further layers grown atop the p-typelayer(s), so that the undoped layer mitigates dopant diffusion into thefurther layers.

In one or more embodiments, a gain medium includes a p-type layer, anundoped layer, a region that includes multiple quantum wells (MQWs), andan n-type layer. The p-type layer defines a first surface and anopposing second surface, and is doped using zinc. The undoped layermaterial defines a first surface and an opposing second surface that areseparated by a thickness of the undoped layer, the thickness beingwithin a thickness range of 5 nm to 500 nm. The first surface of theundoped layer adjoins the second surface of the p-type layer. The regionthat includes the MQWs defines a first surface and an opposing secondsurface that are separated by a thickness of the region. A first surfaceof the region adjoins the second surface of the undoped layer. Then-type layer defines a first surface and a second surface that areseparated by a thickness of the n-type layer, the first surface of then-type layer adjoining the second surface of the region. The p-typelayer, the undoped layer, the region and the n-type layer are formed ofone or more compound semiconductor materials that are characterized byepitaxially compatible crystalline lattice structures. The p-type layer,the undoped layer, the region and the n-type layer form a singlecrystalline structure that extends continuously from the p-type layerthrough the n-type layer.

In one or more embodiments, a method of fabricating a gain mediumincludes growing a p-type layer doped with zinc on a substrate, growingan undoped layer comprising one or more of InP, InGaAsP, InGaAlAs, orInGaAlAsP on the p-type layer, growing a region that includes MQWs onthe undoped layer, and growing an n-type layer on the region. Growingthe undoped layer includes growing the undoped layer to a thickness thatis sufficient to prevent Zn diffusion from the p-type layer into theregion during subsequent growth and/or wafer fabrication steps.

In one or more embodiments, a compound semiconductor layer stack forfabrication of an electronic device includes one or more p-type layers,formed of crystalline, compound semiconductor material grown on acrystalline substrate. At least one of the one or more p-type layersincludes Zn as a dopant. The compound semiconductor layer stack alsoincludes an undoped layer, formed of crystalline, compound semiconductormaterial grown on the one or more p-type layers, and one or more n-typelayers, formed of crystalline, compound semiconductor material grown onthe undoped layer. The undoped layer includes InP, InGaAsP, InGaAlAs, orInGaAlAsP, and is of sufficient thickness to reduce zinc diffusion fromthe p-type layer toward the n-type layers during growth of the one ormore n-type layers.

Further areas of applicability of the present disclosure will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating various embodiments, are intended for purposes ofillustration only and are not intended to necessarily limit the scope ofthe disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are described in detail below with reference to thefollowing figures, in which like numerals within the drawings andmentioned herein represent substantially identical structural elements.

FIG. 1 illustrates a gain chip that is independently fabricated, thenbonded into a substrate in an “n-up” configuration, and coupled withwaveguides, e.g., as part of forming a solid state laser, according toone or more embodiments.

FIG. 2 shows a schematic cross-section of exemplary layers in a “p-up”as-grown configuration, as later used for the gain chip in FIG. 1,according to one or more embodiments.

FIG. 3 shows a schematic cross-section of exemplary layers in an “n-up”as-grown configuration, according to one or more embodiments.

FIG. 4 shows a Secondary Ion Mass Spectroscopy (SIMS) analysis of a gainchip layer stack grown according to the cross-section of FIG. 3,according to one or more embodiments.

FIG. 5 shows a schematic cross-section of exemplary layers in a modified“n-up” as-grown configuration, according to one or more embodiments.

FIG. 6 shows a SIMS analysis of a gain chip layer stack grown accordingto the cross-section of FIG. 5, according to one or more embodiments.

FIG. 7 is a plot of output light vs. input current illustratingperformance of otherwise identical devices with a Zn doped spacer and anundoped spacer, respectively, according to one or more embodiments.

FIG. 8 is a flowchart of a method 750 of fabricating a gain medium,according to one or more embodiments.

DETAILED DESCRIPTION

The subject matter of embodiments of the present invention is describedhere with specificity to meet statutory requirements, but thisdescription is not intended to limit the scope of the claims. Theclaimed subject matter may be embodied in other ways, may includedifferent elements or steps, and may be used in conjunction with otherexisting or future technologies. This description should not beinterpreted as implying any particular order or arrangement among orbetween various steps or elements except when the order of individualsteps or arrangement of elements is explicitly described. Each exampleis provided by way of illustration and/or explanation, and not as alimitation. For instance, features illustrated or described as part ofone embodiment may be used on another embodiment to yield a furtherembodiment. Upon reading and comprehending the present disclosure, oneof ordinary skill in the art will readily conceive many equivalents,extensions, and alternatives to the specific, disclosed luminaire types,all of which are within the scope of embodiments herein.

In the following description, positional terms like “above,” “below,”“vertical,” “horizontal” and the like are sometimes used to aid inunderstanding features shown in the drawings as presented, that is, inthe orientation in which labels of the drawings read normally. Thesemeanings are adhered to, notwithstanding that items described below(e.g., wafers, layer stacks, gain chips, photonic chips) could beinverted during fabrication and/or in subsequent uses.

In certain applications, such as photodiodes, or light-emitting devicesof III-V materials that include multiple quantum well (MQW) regions, Znor other p-type dopant contamination can impair performance and/orreliability. For example, in lasers, such dopants can act asnon-radiative recombination centers in laser active regions. Over time,build-up can occur due to device bias, and can cause reliabilityproblems. In photodiodes, photons generate electron-hole pairs in adepletion region, created by reverse biasing the semiconductor material,to provide electrical signals. Dopant migration into otherwise notintentionally doped areas can reduce the width of the depletion regionunder reverse bias, reducing responsivity to photons.

Certain embodiments herein are based on an appreciation that gainstructures for solid state light emitter and/or laser applications mayadvantageously use Zn, Cd or Mg as p-type dopants in some layers, butmay also employ strategies to limit migration of the dopant to otherlayers, for example MQWs of the gain structures, undoped regions ofphotodiodes and the like. For example, in some applications, a gain chipmay be integrated with reflectors to form a solid state laser. It may beadvantageous to modulate the gain chip at high speed to enable acorrespondingly high bandwidth for communication applications. Suchapplications can benefit from electrical paths to the MQW regions beingas limited as possible, because the currents required to increase anddecrease light output from the MQW regions are at least somewhatproportional to the volume of the semiconductor material in the currentpath.

FIG. 1 illustrates a portion of a photonic chip, including a gain chip100 that is independently fabricated, then coupled with a substrate 10,waveguides 30 and interconnect metal 80, e.g., as part of forming thephotonic chip (e.g., a solid state laser). Gain chip 100 may be formedof a compound semiconductor material, and may include MQW region 101 andother structures, not shown in FIG. 1 due to the scale of the drawing,but described below. As known in the art, light can be generated withinMQW region 101 and can transmit therefrom into waveguides 30. Gain chip100 and the further structures shown in FIG. 1 are provided as just oneillustration of use of a gain chip and do not limit the applicability ofthe concepts described further below. Upon reading and comprehending thepresent disclosure, one of ordinary skill in the art will readilyconceive many equivalents, extensions, modifications, adaptations andalternatives to the specific structure shown in FIG. 1.

Gain chip 100 may be integrated with the surrounding structures andcomponents as shown in FIG. 1 using techniques described in U.S. Pat.No. 9,316,785, and examples of solid state lasers and/or other systemsthus formed may be found in U.S. Pat. No. 9,664,855. These patents areincorporated by reference in their entireties herein, for all purposes,and are further referred to as “the Incorporated Patents.”

To generate the structure shown in FIG. 1, substrate 10 may be preparedby adding materials corresponding to a first cladding layer 20,waveguides 30, and a second cladding layer 40. Substrate 10, claddinglayers 20 and 40, and waveguides 30 extend to regions beyond those shownin the photonic chip portion illustrated in FIG. 1. A recess may beetched into one or more of these materials, and optionally to some depthwithin substrate 10. A bonding material 70 may be formed within therecess, or may be formed atop gain chip 100 before it is flipped,aligned with the recess, and bonded using material 70. Spacers 50 mayoptionally be formed to about the height of cladding 20, and indexmatching elements 60 may optionally be formed between waveguides 30 andgain chip 100. Interconnect metal 80 may be formed atop gain chip 100,and may extend to other structures not shown in the photonic chip regiondepicted in FIG. 1. A passivation material 90 may be applied atop allstructures of the photonic chip, and may be planarized (e.g., usingchemical mechanical planarization) as shown in FIG. 1, or may be left inits as-deposited state without planarization. The processes discussedabove and in the Incorporated Patents are generally derived fromprocesses utilized in semiconductor fabrication, includingphotolithography, etching, depositions and the like. Upon reading andcomprehending the present disclosure, one of ordinary skill in the artwill readily conceive many equivalents, extensions, modifications,adaptations and alternatives.

Also, as is common in the art, key layers of gain chip 100 are first“grown” in wafer form using molecular beam epitaxy (MBE), metallorganicchemical vapor deposition (MOCVD) or the like. (The term “grown” isunderstood in the art to mean that successive layers of materialscharacterized by compatible crystalline lattice structures are formed ona substrate with a crystalline structure, and that the materials arrangethemselves into position with respect to the crystalline structure toform layers that continue the original crystalline lattice.) Then, thewafer may be singulated into individual gain chips that are temporarilybonded to a transfer substrate, flipped, and individually transferred tothe substrate shown in FIG. 1. Thus, either the wafer with the grownlayers, or any of the individual chips formed therefrom, may beconsidered a gain medium.

In compound semiconductor gain chip 100, Si may be used as an n-typedopant for one set of layers (e.g., those layers shown above MQW region101), while Zn, Cd or Mg may be used as p-type dopants for certain otherlayers. The assembled configuration is inverse to the sequence in whichthe layers are grown during MBE or MOCVD, because gain chip 100 isflipped as it transfers to the bonding site of a host substrate in whichthe photonic chip is integrated. Thus, layers of gain chip 100 to beused in a finished, “n-up” product configuration are actually grownstarting with the n-layers at the “bottom” of the stack (e.g., nearestto the growth substrate), that is, in an as-grown, “p-up” configuration.

A schematic cross-section of exemplary layers in an as-grown, “p-up”configuration 200 is shown in FIG. 2. Certain ones of the layers shownin as-grown, “p-up” configuration 200 are present to facilitateelectrical contact, optical confinement and/or fabrication needs. Thus,many of these layers could be modified or omitted, and/or further layerscould be added, depending at least on the desired final product, and thewafer fabrication tools available. Intentionally n- and p-type dopedregions are indicated with different fill patterns; the MQW regions andimmediately surrounding layers are not intentionally doped.Configuration 200 could be grown in wafer form and then singulated toform gain chip 100, FIG. 1. Configuration 200 begins with a compoundsemiconductor substrate 210 upon which n-type layers, a MQW region, andp-type layers are grown. In order, the exemplary layers of as-grown,“p-up” configuration 200 include an n contact layer 220, an n bufferlayer 225, an n etch stop layer 230, an n spacer layer 235, an undopedconfinement layer 245, MQWs 250, another undoped confinement layer 255,a p spacer layer 260, a p etch stop layer 265, a p buffer layer 270 anda p contact layer 275.

Recent study has shown that a finished, “p-up” product configuration canbe preferable for high speed operation of a finished laser or otherdevice. In the case of lasers, this is because the typically lowermobility of holes in the p-type contact, buffer and other layers, tendsto confine current introduced through contacts at the top of the gainchip enroute to the MQWs, reducing the volume of semiconductor materialto be modulated. This increases the maximum bandwidth of a laser orother device formed using gain chip 100. The “p-up” productconfiguration requires that the gain chip be grown in an as-grown,“n-up” configuration, because of the flipping required to integrate thegain chip with the other solid-state laser components.

A schematic cross-section of exemplary layers in a first as-grown,“n-up” configuration 300 is shown in FIG. 3. Configuration 300 could begrown in wafer form and then singulated to form gain chip 100, FIG. 1.Configuration 300 begins with a compound semiconductor substrate 310(which could be the same material as substrate 210, FIG. 2, since mostor all of substrate 310 may be removed during the integration processfor the photonic chip shown in FIG. 1). Upon substrate 310, p-typelayers, an optional MQW region, and n-type layers are grown. In order,the exemplary layers of as grown, “n-up” configuration 300 include a pcontact layer 320, a p buffer layer 325, a p etch stop layer 330, a pspacer layer 335, an undoped confinement layer 345, optional MQWs 350,another undoped confinement layer 355, an n spacer layer 360, an n etchstop layer 365, an n buffer layer 370 and an n contact layer 375.Certain ones of the layers shown in as-grown, “n-up” configuration 300are present to facilitate electrical contact, optical confinement and/orfabrication needs. Thus, many of these layers could be modified oromitted, and/or further layers could be added, depending at least on thedesired final product, and the wafer fabrication tools available.

In the present discussion, the term “undoped” shall be understood tomean “not intentionally doped,” that is, “undoped” does not precludenormal background or contaminant levels of elements that are normallyfound in most or all semiconductors. “Undoped” materials herein may haveactual concentrations of a given dopant that are generally less thanabout 5×10{circumflex over ( )}16 atoms/cm{circumflex over ( )}3,usually less than about 1×10{circumflex over ( )}16 atoms/cm{circumflexover ( )}3, and possibly less than about 5×10{circumflex over ( )}15atoms/cm{circumflex over ( )}3.

Growing the gain chip in the as-grown, “n-up” configuration, and usingZn, for example, as the p-type dopant, requires introducing asignificant quantity of Zn into the spacer, etch stop and buffer layersthat are grown early in the gain chip layer stack. Thus, Zn is alreadypresent in the lower layers while the optional MQWs and upper, n-typecontact layers are grown. Because of the relatively high temperaturesinvolved with layer growth (and/or other wafer fabrication steps) andthe concentration of Zn throughout the p-type layers (layers 320, 325,330 and 335) significant diffusion of Zn can occur, with the Zn movingupwards from the p-type layers, toward and possibly into or throughoptional MQWs 350. Similar effects can also occur when other elements,such as Cd or Mg, are used for the p-type dopant.

FIG. 4 shows a secondary ion mass spectroscopy (SIMS) analysis 400 of aportion of a gain chip layer stack that was grown according to thecross-section of FIG. 3. A solid line 401 indicates galliumconcentration, and a broken line 402 indicates zinc concentration, bothas a function of depth. The concentration (vertical) scale is inarbitrary units with each broken horizontal line representing a factorof 10 in concentration. The depth (horizontal) scale is also inarbitrary units, with the left hand side of FIG. 4 corresponding to anuppermost surface of the analyzed region, and depth within the sampleincreasing to the right. The region analyzed in FIG. 4 begins within then-buffer layer and extends into the p-buffer layer, as defined in FIG.3. A depth range 410 corresponds to the approximate depth of MQWs 350,and another depth range 420 corresponds to the approximate depth of pspacer layer 335, as identified in FIG. 3. A region 430 in which ameasurable concentration of Zn exists, extending from part of p-spacerlayer up through and slightly above MQWs 350, is noted. The amount of Znfound in region 430 is undesirable.

In an improved version of the structure illustrated in FIGS. 3 and 4, Znmigration was significantly reduced by changing p spacer layer 335,between the p-doped layers and MQWs 350 in FIG. 3, to an undoped spacerlayer. In the present discussion, the term “undoped” shall be understoodto mean “not intentionally doped,” that is, “undoped” does not precludenormal background or contaminant levels of elements that are normallyfound in most or all semiconductors. “Undoped” materials herein may haveactual concentrations of a given dopant that are generally less thanabout 2×10{circumflex over ( )}16 atoms/cm{circumflex over ( )}3,usually less than about 1×10{circumflex over ( )}16 atoms/cm{circumflexover ( )}3, and possibly less than about 5×10{circumflex over ( )}15atoms/cm{circumflex over ( )}3. It is also emphasized that althoughstructures including MQWs and use of Zn as a dopant are discussed hereinas examples, other structures may benefit from blocking the diffusion ofdopants, and the principles discussed below apply equally to structuresthat do not include MQWs, and/or use dopants other than Zn.

Compared to the p spacer layer, the undoped spacer layer both reducesthe amount of dopant initially placed near to sensitive regions such asMQWs, and forms a diffusion barrier to the underlying doped layersduring subsequent high temperature processing. In many epitaxial andfabrication sequences, most high temperature processing occurs in theepitaxial processing itself. In conventional laser structures, then-type layers are grown first and the p-type layers are grown second, sothat the p-type regions are subject to high growth temperatures for onlya short period. For structures in which the p-type layers are grownfirst, the p-type layers are exposed to high temperatures for a longertime and may experience significant Zn (or Cd or Mg) diffusion ingressduring the epitaxial processing alone. Therefore, there may berelatively little reason to add an undoped spacer layer when theas-grown configuration grows the n-type layer(s) first and the p-typelayer(s) second.

Materials that can be used for an undoped spacer layer can be determinedby characteristics such as compatibility with the lattice structure ofthe underlying substrate and/or overlying layers, low diffusivity of theselected dopant at MBE/MOCVD and other wafer fabrication temperatures,high bandgap, and sufficient current injection efficiency. In lasers,current injection efficiency is important because the modulated currentgoing to the MQWs will pass through the layer. The minimum bandgap isthat which will make the spacer layer transparent to light beinggenerated by, or otherwise associated with, the final product. Specificvalues of these characteristics can easily be determined by one skilledin the art for possible diffusion blocking layer materials without undueexperimentation, given knowledge of the lattice structure of thesubstrate, the wavelength of the light intended to be generated by, orotherwise associated with operation of, a photonic chip into which thegain medium will be integrated, and the likely times and temperatures ofMBE/MOCVD growth and/or subsequent fabrication steps. For example, gooddiffusion blocking layer materials for InP related compounds includeInP, InGaAsP, InGaAlAs, and InGaAlAsP.

Experimentally, it has been validated that an undoped InP layer providesa useful barrier to such Zn migration within a layer stack grown on anInP substrate. When intermediate layers grown on an InP substrateinclude mixtures of Al, Ga and/or In as group III materials, and Asand/or P as group V materials, InGaAsP, InGaAlAs, or InGaAlAsP can alsobe used as the undoped layer. It is believed that layers up to about 500nm thick, and thicker, would be useful, although thick layers wouldincrease the volume of adjacent regions such as regions of MQWs. Anundoped spacer layer as thin as 25 or 50 nm, or even less, possibly downto 5 nm, may also be effective in at least reducing Zn or other dopantssomewhat in adjacent regions. Thus, possible nominal layer thicknessesfor an undoped InP, InGaAsP, InGaAlAs, or InGaAlAsP spacer layer toreduce dopant diffusion include 5, 10, 20, 25, 30, 35, 40, 50, 60, 75,85, 100, 110, 125, 135, 150, 175, 200, 250, 300, 350, 400, 450 and 500nm, and thicknesses intermediate to these values. Maximum thickness ofan undoped spacer layer can be determined as a tradeoff amongeffectiveness at blocking dopant diffusion (thicker is better), cost ofmaterial/epitaxial processing time (thinner is better), device effectssuch as series resistance through the layer (thinner is better), andvarious, application-specific process integration issues such as totallayer stack height, effect on MQW placement within a layer stack,effects on etching through a layer stack, and the like (various onescould weigh towards thicker or thinner). For an exemplary process offabricating a gain medium, maximum layer height was determined as about125 nm, so as to substantially block Zn diffusion, with minimal materialcost and a minimal increase in series resistance.

A schematic cross-section of exemplary layers for the modified,as-grown, “n-up” configuration 500 is shown in FIG. 5. Configuration 500could be grown in wafer form and then singulated to form gain chip 100,FIG. 1. Configuration 500 begins with a compound semiconductor substrate510 (which could be the same material as substrates 210/310, FIGS. 2,3). Upon substrate 510, p-type layers, an optional MQW region, andn-type layers can be grown. In order, the exemplary layers of as grown,“n-up” configuration 500 include a p contact layer 520, a p buffer layer525, a p etch stop layer 530, an undoped spacer layer 540, an undopedconfinement layer 545, optional MQWs 550, another undoped confinementlayer 555, an n spacer layer 560, an n etch stop layer 565, an n bufferlayer 570 and an n contact layer 575. Similarly to configurations 200and 300, certain ones of the layers shown in as-grown, “n-up”configuration 500 are present to facilitate electrical contact, opticalconfinement and/or fabrication needs and could be modified or omitted,and further layers could be added.

FIG. 6 shows a SIMS analysis 600 of a portion of a gain chip layer stackgrown with an undoped spacer on the p-side, according to thecross-section of FIG. 5. Similar to those shown in FIG. 4, a solid line601 indicates gallium concentration, and a broken line 602 indicateszinc concentration, both as a function of depth. The concentration(vertical) scale is in arbitrary units with each broken horizontal linerepresenting a factor of 10 in concentration. The depth (horizontal)scale is also in arbitrary units, with the left hand side of FIG. 6corresponding to an uppermost surface of the analyzed region, and depthwithin the sample increasing to the right. The region analyzed in FIG. 6begins within the n-buffer layer and extends into the p-buffer layer, asdefined in FIG. 5. A depth range 610 corresponds to the approximatedepth of MQWs 550. In FIG. 6, an identified region on the p-side is thatof undoped spacer layer 540. It can be seen that the concentration of Znat depths corresponding to p buffer layer 525 and contact layer 520 aresimilar to those in SIMS analysis 400, FIG. 4, but the concentration ofZn in a region 630 around MQWs 550 is reduced by about an order ofmagnitude, compared with the corresponding region in SIMS analysis 400.The reduced Zn concentration around MQWs 550 is expected to provide areliability improvement for devices fabricated according to n-upconfiguration 500, FIG. 5, as compared to devices fabricated accordingto n-up configuration 300, FIG. 3.

FIG. 7 is a plot of output light vs. input current illustratingperformance of otherwise identical devices with a Zn doped spacer and anundoped spacer, respectively (e.g., devices with the cross-sectionalprofiles illustrated in FIGS. 3 and 5, respectively). The tested deviceswere configured as gain chips for lasers. Results for the devices withundoped spacers are shown in solid lines, and results for the deviceswith Zn doped spacers are shown in broken lines. Laser thresholds andslope efficiencies for the tested devices with undoped spacer layers areshown to be at least equivalent to those with p-doped spacer layers.

FIG. 8 is a flowchart of a method 750 of fabricating a gain medium.Steps of method 750 can be used, for example, to fabricate gain chip100, FIG. 1, or the exemplary layers for the as-grown, “n-up”configuration 500 shown in FIG. 5. The illustrated steps of method 750include steps that are optional, and it should be understood that method750 may be performed with or without such steps, as well as performingcertain steps in a different order than is shown, as discussed below.

Step 752 of method 750 grows one or more p-type layers on a compoundsemiconductor substrate, for example using Zn, Cd or Mg as a dopant.Examples of step 752 include growing p contact layer 520, p buffer layer525 and/or p etch stop layer 530, as illustrated in FIG. 5. Growing pcontact layer 520 and p buffer layer 525 are illustrated in FIG. 8 asoptional substeps 754 and 756.

Step 760 of method 750 grows an undoped buffer layer of sufficientthickness to reduce dopant diffusion from the p-type layers duringsubsequent growth and/or wafer fabrication steps. An example of step 760is growing undoped spacer layer 540, FIG. 5. As discussed above, thelayer grown in step 760 must be epitaxially compatible with underlyingand overlying layers, and knowledge of dopant diffusivity for the layergrown in step 760 enables one skilled in the art to determine athickness of the undoped spacer to ensure that dopant diffusion issufficiently blocked during subsequent growth and/or wafer fabricationsteps.

In an optional step 770, a region that includes MQWs is grown. Inanother optional step 780, further undoped layers are grown. Theinclusion, order and repetition of optional steps 770 and/or 780 areoptional to method 750, and are determined according to the finalapplication of the layer stack being grown. For example, for gain chipapplications, optional MQWs (step 770) may be desired, and it may bedesirable to grow confinement layers (step 780) above and below theMQWs. Another example of step 780 could be growing a thick undopedlayer, without MQWs, for use in a photodiode application.

In step 790, n-type layers are grown. Examples of step 790 are growing nspacer layer 560, n etch stop layer 565, n buffer layer 570 and/or ncontact layer 575, as illustrated in FIG. 5. Growing n buffer layer 570and n contact layer 575 are illustrated in FIG. 8 as optional substeps792 and 794.

One further optional steps of method 750 is a step 795 of singulatingthe substrate on which the above-noted layers have been grown, to formindividual chips such as gain chips, photodiodes and the like. Anotherfurther optional step of method 750 is a step 797 of integrating thelayer stack grown into a photonic chip. Depending on the desiredapplication, steps 795 and 797 may be done in either order and/orrepeated. An example of steps 795 and 797 performed in sequence issingulating a wafer into individual gain chips, and integrating one ormore of the gain chips into photonic chips (e.g., as shown schematicallyin FIG. 1). An example of step 797 performed alone is adding furtherfabrication steps before and/or after the other steps of method 750, tofabricate layer stacks into photonic devices (e.g., light emittingdiodes, edge emitters, vertical cavity surface emitting lasers (VCSELs)and/or photodiodes), or other diodes or transistors, in wafer form. Oneskilled in the art will readily conceive many equivalents, extensions,modifications, adaptations and alternatives.

Therefore, the above-described techniques of providing an undoped layerbetween an underlying doped layer and overlying layers can be applied tolayer stacks used to fabricate many products. Compound semiconductorproducts that are fabricated from layer stacks grown in an “n-up”configuration (e.g., with undoped and/or n-type layers grown after ap-type, Zn, Cd or Mg doped layer) are expected to benefit similarly fromaddition of an undoped layer, or substitution of an undoped layer for adoped layer, to block dopant diffusion from undoped and/or p-typelayers. Products that may benefit from an undoped layer to block dopantdiffusion may include, but are not limited to, gain media, lightemitting diodes, edge emitter lasers, VCSELs, modulators, photodiodes,power diodes, transistors and the like. Appropriate thickness andmaterial type of a useful undoped layer can be determined by consideringthe thermal profile of the growth operation and subsequent fabricationsteps, diffusivity of dopant in the undoped layer and adjacent layersthroughout the expected growth and fabrication thermal profiles, latticecompatibility, bandgap, and/or current injection efficiency. Compoundsemiconductor layer stacks thus grown may include III-V or II-VIsemiconductors.

The foregoing is provided for purposes of illustrating, explaining, anddescribing embodiments of the present invention. Upon reading andcomprehending the present disclosure, one of ordinary skill in the artwill readily conceive many equivalents, extensions, modifications,adaptations and alternatives. These equivalents, extensions,modifications, adaptations and alternatives may be made withoutdeparting from the scope or spirit of the invention. Differentarrangements of the components depicted in the drawings or describedabove, as well as components and steps not shown or described, arepossible. Similarly, some features and subcombinations are useful andmay be employed without reference to other features and subcombinations.

The above description of exemplary embodiments of the invention has beenpresented for the purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdescribed, and many modifications and variations are possible in lightof the teaching above. Embodiments of the invention have been describedfor illustrative and not restrictive purposes, and alternativeembodiments will become apparent to readers of this patent. Theembodiments were chosen and described in order to explain the principlesof the invention and practical applications thereof, to enable othersskilled in the art to best utilize the invention in various embodimentsand with various modifications as are suited to the particular usecontemplated. Accordingly, the present invention is not limited to theembodiments described above or depicted in the drawings, and variousembodiments and modifications can be made without departing from thescope of the claims below.

The specific details of particular embodiments may be combined in anysuitable manner without departing from the spirit and scope ofembodiments of the invention. However, other embodiments of theinvention may be directed to specific embodiments relating to eachindividual aspect, or specific combinations of these individual aspects.

Also, it is noted that the embodiments may be described as a processwhich is depicted as a flowchart, a flow diagram, a data flow diagram, astructure diagram, or a block diagram. Although a flowchart may describethe operations as a sequential process, many of the operations can beperformed in parallel or concurrently. In addition, the order of theoperations may be re-arranged. A process is terminated when itsoperations are completed, but could have additional steps not includedin the figure. A process may correspond to a method, a function, aprocedure, a subroutine, a subprogram, etc.

A recitation of “a”, “an”, or “the” is intended to mean “one or more”unless specifically indicated to the contrary. All patents, patentapplications, publications, and descriptions mentioned here areincorporated by reference in their entirety for all purposes. None isadmitted to be prior art.

What is claimed is:
 1. A gain medium, comprising: a p-type layer thatdefines a first surface and an opposing second surface, the p-type layerbeing doped using zinc; an undoped layer that defines a first surfaceand an opposing second surface that are separated by a thickness of theundoped layer, the thickness being within a thickness range of 5 nm to500 nm, the first surface of the undoped layer adjoining the secondsurface of the p-type layer; a region that includes multiple quantumwells (MQWs), wherein the region defines a first surface and an opposingsecond surface that are separated by a thickness of the region, and afirst surface of the region adjoins the second surface of the undopedlayer; and an n-type layer that defines a first surface and a secondsurface that are separated by a thickness of the n-type layer, the firstsurface of the n-type layer adjoining the second surface of the region;wherein: the p-type layer, the undoped layer, the region and the n-typelayer are formed of one or more compound semiconductor materials thatare characterized by epitaxially compatible crystalline latticestructures; and the p-type layer, the undoped layer, the region and then-type layer form a single crystalline structure that extendscontinuously from the p-type layer through the n-type layer.
 2. The gainmedium of claim 1, further comprising a substrate characterized by theepitaxially compatible crystalline lattice structure, and wherein thep-type layer, the undoped layer, the region and the n-type layersuccessively adjoin the substrate.
 3. The gain medium of claim 2,wherein the p-type layer, the undoped layer, the region and the n-typelayer are successively grown on the substrate using one of metallorganicchemical vapor deposition and molecular beam epitaxy.
 4. The gain mediumof claim 1, wherein the thickness of the undoped layer is within a rangeof 25 to 125 nm.
 5. The gain medium of claim 1, wherein the p-typelayer, the undoped layer, the region and the n-type layer form part of asemiconductor wafer.
 6. The gain medium of claim 1, wherein the p-typelayer, the undoped layer, the region and the n-type layer form a gainchip.
 7. The gain medium of claim 6, wherein the gain chip is integratedwithin a photonic chip.
 8. The gain medium of claim 7, wherein a bandgapof the undoped layer is high enough to make the undoped layertransparent to light associated with operation of the photonic chip. 9.The gain medium of claim 1, wherein the p-type layer, the region and then-type layer are grown on an InP crystalline lattice, and the undopedlayer includes one or more of InP, InGaAsP, InGaAlAs, or InGaAlAsP. 10.A method of fabricating a gain medium, comprising: growing a p-typelayer doped with zinc on a substrate; growing an undoped layercomprising one or more of InP, InGaAsP, InGaAlAs, or InGaAlAsP on thep-type layer; growing a region that includes multiple quantum wells(MQWs) on the undoped layer; and growing an n-type layer on the region;wherein growing the undoped layer comprises growing the undoped layer toa thickness that is sufficient to prevent Zn diffusion from the p-typelayer into the region during subsequent growth or wafer fabricationsteps.
 11. The method of claim 10, wherein each of the steps of growingcomprises using metallorganic chemical vapor deposition or molecularbeam epitaxy.
 12. A compound semiconductor layer stack, comprising: oneor more p-type layers, formed of crystalline, compound semiconductormaterial grown on a crystalline substrate, wherein at least one of theone or more p-type layers comprises Zn as a dopant; an undoped layer,formed of crystalline, compound semiconductor material grown on the oneor more p-type layers; and one or more n-type layers, formed ofcrystalline, compound semiconductor material grown on the undoped layer;wherein the undoped layer comprises InP, InGaAsP, InGaAlAs, orInGaAlAsP, and is of sufficient thickness to reduce zinc diffusion fromthe p-type layer toward the n-type layers during growth of the one ormore n-type layers.
 13. The compound semiconductor layer stack of claim12, wherein the undoped layer defines a thickness in the range of 5 nmto 500 nm.
 14. The compound semiconductor layer stack of claim 12,wherein the crystalline substrate is a compound semiconductor wafer. 15.The compound semiconductor layer stack of claim 12, wherein thecrystalline substrate is an individual chip singulated from a compoundsemiconductor wafer.
 16. The compound semiconductor layer stack of claim15, wherein the compound semiconductor wafer is singulated to form aplurality of gain chips, and at least one of the gain chips isintegrated with one or more waveguides within a photonic chip.
 17. Thecompound semiconductor layer stack of claim 12, wherein furtherfabrication steps are applied to the layer stack, to form one or more ofa light emitting diode, an edge emitter laser, a vertical cavity surfaceemitting laser, a modulator, a photodiode, or a power diode.
 18. Themethod of claim 10, wherein growing the undoped layer comprises limitingthe undoped layer to a thickness between 5 nm and 500 nm.
 19. The methodof claim 10, wherein the substrate is a compound semiconductor wafer,and further comprising singulating the compound semiconductor wafer toform a plurality of gain chips.
 20. The compound semiconductor layerstack of claim 16, wherein a bandgap of the undoped layer is high enoughto make the undoped layer transparent to light associated with operationof the photonic chip.